Spermatogonial stem cells (SSCs) are adult issue stem cells capable of producing all of the germ cells in the spermatogenic lineage. Unlike pluripotent embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs), which can produce any/all cells in the body, SSCs only contribute to male germ cell production (under most circumstances). SSCs can be manipulated ex vivo using a culture system and evaluated for their biological capacity to produce and maintain spermatogenesis using a functional transplantation assay where they are placed in the seminiferous tubules of infertile recipients and regenerate spermatogenesis. We study the fundamental processes that allow these cells to produce the spermatogenic lineage and the potential utility of these cells in the clinic.